Vortex Shedding From Low Aspect Ratio Dual Step Cylinders

Abstract

A low aspect ratio dual-step cylinder is comprised of two cylinders of different diameters (D and d). The large diameter cylinder (D) with low aspect ratio (L/D) is attached to the mid-span of a small diameter cylinder (d). This geometry is relevant to many engineering applications, e.g., finned-tube heat exchangers, underwater cables, and cylindrical support structures. The present study investigates the effect of Reynolds number (ReD) and L/D on dual step cylinder wake development for 1050 ≤ ReD ≤ 2100, D/d = 2, and 0.2 < L/D ≤ 3. Experiments have been performed in a water flume facility utilizing flow visualization, Laser Doppler Velocimetry (LDV), and Particle Image Velocimetry (PIV). The results show that vortex shedding occurs from the large and small diameter cylinders at distinct frequencies for L/D ≥ 1 & ReD = 2100 and L/D ≥ 2& ReD = 1050. At these higher aspect ratios investigated, large cylinder vortices predominantly form closed vortex loops in the wake and small cylinder vortices form half-loop vortex connections. In contrast, at lower aspect ratios, vortex shedding from the large cylinder ceases, with the dominant frequency centred-activity in the large cylinder wake attributed to the passage of vortex filaments connecting small cylinder vortices. The presence of the large cylinder distorts the vortex filaments causing cyclic vortex dislocations accompanied by the formation of half-loop vortex connections. Increasing L/D decreases the frequency of occurrence of vortex dislocations and increases the dominant frequency in the large cylinder wake. The results also show that the Reynolds number has a substantial effect on wake vortex shedding frequency, which is more profound than that expected for a uniform cylinder.